Online colour grading LUTs
You absolutely nailed it; they are a load of codswallop and rubbish.

The bottom line is that all colour transforms are based on a chain of assumptions. The moment a single assumption is wrong, or a single transform is out of order, the whole stack becomes invalid.

TL;DR is you can trust the transforms if you know they are the correct ones and they are / were applied properly. However, I would be at least as skeptical with purchased LUTs as one should be with HDRIs; prepare to be underwhelmed at best, and outright scammed at worst.

Fantastic question and welcome to the forum.

;)